The invention concerns a method for anchoring a joining element in an object and a joining element to be used in the method. The joining element comprises a thermoplastic material (or a material having a thermoplastic component) and anchoring is effected with the aid of the thermoplastic material and mechanical vibration (e.g. ultrasonic vibration) through which the thermoplastic material is at least locally liquefied. The joining element serves for connecting a further object to the named object in which it is anchored or the joining element is part of such further object.
It is known from the publications WO 98/42988 and WO 00/79137, to anchor joining elements in objects with the aid of a thermoplastic material which is liquefied by mechanical vibration, and which, in the liquefied state, is pressed into suitable surface structures in the object, where on re-solidification it forms a positive fit connection with the named surface structures and therewith anchors the joining element in the object. The material of the object in which the joining element is to be anchored possesses e.g. an open porosity or the surface of the object in a region where anchorage is to be affected comprises a plurality of corresponding cavities or protrusions. The thermoplastic material of the joining element usually has an elasticity module which is greater than 0.5 GPa and therefore this material can be liquefied by mechanical vibration only if contact with the object is limited to a plurality of small locations (energy directors). This is achieved e.g. by providing energy directors in the form of elements protruding from a main surface of the joining element which protruding elements taper to an edge or tip.
In many cases in which a joining element is anchored in an object with the aid of a thermoplastic material and mechanical vibration, an opening is provided in the object and it is desired that the joining element is anchored either in the walls of the opening or in the walls and in the bottom of the opening. Anchorage in the walls of the opening is achieved usually by dimensioning the joining element and the opening such that the circumference of a cross section of the joining element including energy directors is somewhat larger than the corresponding circumference of the cross section of the opening, such that on introducing the joining element into the opening, the circumference of the joining element is at least locally (energy directors) pressed against the wall of the opening.
In particular, if anchorage in the walls of the opening is desired in the immediate vicinity of the mouth of the opening, it often happens that the liquefied material interpenetrates the surface of the object in the vicinity of the opening and small amounts of the material protrude onto this surface. This effect blurs the contours of the opening and/or of the joining element in the mouth region such that these contours do not appear as clear cut as this may be desired for various reasons. In particular, if a proximal face of the joining element, which is anchored in an opening provided in the object, is to be flush with the object surface it is desirable mainly for esthetic reasons that the circumference of the joining element and of the opening are clearly defined, i.e. not blurred at all.
Anchorage in the immediate vicinity of the mouth of the opening is in many cases desired and it is necessary, if the object is very thin or if the opening is very shallow. This is the case if the object is a sandwich board with thin outer layers and an inner layer, wherein anchorage is possible only in the outer layers. An example of such a board is a so called hollow core board comprising outer layers of wood or a wood-like material and an inner layer being constituted by a honeycomb structure of cardboard, in which the named anchorage is not possible.